\WP_Stager_Integration\Calendar::calculate_timestamp_day_start(int $timestamp): int

Full Code

/**
 * Calculates the start timestamp of a day (00:00:00 of the day in the site’s timezone).
 *
 * @param int   $timestamp  A Unix timestamp to calculate the day start timestamp for
 *
 * @return int  A Unix timestamp for a day's 00:00:00
 */
public static function calculate_timestamp_day_start( $timestamp ) {

    // Type casting
    $timestamp = (int) $timestamp;

    // Get date string
    $date_string = wp_date( 'Y-m-d 00:00:00', $timestamp );

    return self::calculate_local_timestamp( $date_string );
}